1. Field of the Invention
The present invention relates generally to a scrambling system for an audio frequency signal and more particularly is directed to a scrambling system for an audio frequency signal which is suitable for being used in a case wherein successive segments of audio frequency signal are rearranged in an and then transmitted.
2. Description of the Prior Art
As a method for scrambling an audio signal in, for example, a cable television broadcast system, there has been proposed a method wherein a block of an audio signal having a certain length is divided into segments and the rearrangement of these segments is carried out on a compressed timebase. In that case, there is a defect that due to a discontinuity of a waveform at the border between the rearranged segments, the succeeding waveform is partly distorted. To remove this defect, the same assignee of this application has previously proposed a timebase compressing and expanding method of a waveform (which is disclosed in Japanese patent application, No. 222299/1982).
In this previously proposed method, at a scrambling side, a waveform of an audio signal which is a little longer than one segment duration of time is timebase-compressed to one segment duration of time and then transmitted, while at an unscrambling side, a waveform of an audio signal corresponding to the net segment amount in the one segment duration of time is extracted and then timebase-expanded to the original one segment duration of time and the timebase-expanded segments are connected whereby to remove the defect caused by the discontinuity in the waveform. This previously proposed method is quite effective for the restricted analog transmission band region.
In such method, when a vertical synchronizing signal VD or the like in a television video signal is used as the synchronizing control signal, no problem occurs at all under the state that the audio signal and the video signal are synchronized with each other, namely, no time displacement exists therebetween or the time displacement is fixed.
However, in, for example, a transmission system, when the transmission routes or paths of the video signal and the audio signal are different from each other, a relative time displacement of the two signals sometimes occurs at the receiving end or side. Moreover, when the recording and/or reproducing is carried out by a VTR (video tape recorder), a video image is adjusted to be optimum. As a result, these signals are reproduced having the time displacement. Furthermore, a wow and flutter inherent in the VTR itself may sometimes cause the video signal and the audio signal to have a time displacement therebetween. When the synchronization is disordered in the transmission recording and reproducing system as described above, and the sequential order of the segments is rearranged to the original order, the connected portion between the segments is not formed correctly so that the audio signal is distorted at the connected portion or a noise is generated at the connected portion. As a result, there occurs a problem that the quality of the audio signal is deteriorated and so on.